I also spent some time reviewing a colleague's >10KLOC shell application, during which my most repeated (verbal) review comment
was ".. and at what point did you realise that writing this in shell was a really stupid idea?"
I feel your pain.
Sounds like he got you to review the code after it was "done". Is that right?
If so, it looks like an example of the classic Beware of a guy in a room anti-pattern.
He should have been discussing the general approach with the team long before going off
to create his shell script monstrosity.
I see this was back in 2008-2009.
I trust this sort of dysfunctional teamwork would not occur at Google today, given
their seminal gTeams study of 2015/2016
(discussed at Working Solo and in a Team and Psychological Safety).
After being burned by a number of similar incidents, I had a rare success at work,
managing to persuade all of R&D to stop writing Unix shell (and Windows BAT)
monstrosities, based on the arguments put forward in Unix shell versus Perl.